In recent years, in our country, as a matter of course, in all the countries of the world, lack of daily life water (especially drinkable water and water used for food) is posed as a problem. As one of the reasons, the supply of daily life water cannot catch up with an increase in growth of population in the world (mainly developing countries). Furthermore, population flowing from local areas to urban areas is increasing in each of the countries, and, in an urban area which originally has a small number of freshwater supplies such as rivers and lakes, imbalance of supply and demand of daily life water for population occurs. Due to desertification caused by global warming, concentration of heavy rains on specific areas, and the like, freshwater which can be used as daily life water is hard to be pooled. In addition, with raising of environmental awareness, dams have been difficult to be constructed in rivers.
For these reasons, in each of the countries of the world including our country, lack of daily life water becomes a serious problem. The lack of daily life water is an issue which may cause internal conflicts or dispute with neighboring countries sometimes.
In the above circumstances, a salt water desalination equipment that desalinates salt water has been installed in a coast area or the like to obtain freshwater from salt water. Also in our country, salt water desalination equipments installed in a relatively calm inland sea such as a coast or a bay area of an urban area in which lack of water easily occurs have increased in number. The salt water desalination equipment draws salt water from an inland sea and produces freshwater by using components such as a reverse osmosis membrane and a forward osmosis membrane. The produced freshwater is used as daily life water.
Conventional basic techniques about salt water desalination equipments using osmosis membranes have been proposed. The salt water desalination equipments are actually installed in various places. The salt water desalination equipments are practically utilized.
In addition, for a salt water desalination equipment using an osmosis membrane, techniques for improving efficiency of desalination from salt water and flow control for the improvement in efficiency are also proposed (for example, see PTLs 1 and 2). In the salt water desalination equipment, a power consumption in a salt water supply pump for supplying salt water to an osmosis membrane component which desalinates salt water is high. A technique which reduces a load on the salt water supply pump is also proposed (for example, see PTL 3).
In particular, a reduction in power consumption in a salt water desalination equipment has been required in recent years. In terms of the power consumption, especially, a reduction in power consumption in a salt water supply pump is an important problem.